Christian Horner says he believes Mercedes carried out its Pirelli tyre test "in an underhand way" and hasn't complied with the Sporting Regulations.

Red Bull and Ferrari protested against Mercedes' test with Pirelli before the Monaco Grand Prix, with Mercedes then going on to win the race. Horner said the timing of the protest showed just how strongly Red Bull felt about what had occurred and that in his view the rules regarding testing are "very clear" and have been ignored.

"What's wrong is that a team, in an underhand way, consciously tested tyres that were designed for this year's championship," Horner said. "We view that the testing rules are very clear in the Sporting Regulations and when you enter the championship at the start of the year you sign up to those regulations.

"In our opinion, in doing that test, Mercedes hasn't complied and therefore we protested before the race because we wanted it to be clear that it wasn't a question of what result they have and that at the first opportunity we had to put in a protest; we put it in because we want clarity."

With the matter referred to the FIA, Horner emphasised that one the contracts he believes Mercedes to have broken is solely between the teams and will also now need revisiting.

"We talk an awful lot about saving costs and we spent three hours on Friday talking about in-season testing and trying to find a solution and it turns out one team has already done a huge amount of it. There also is a testing agreement, that may as well be declared null and void now, which is a contract between the teams that isn't governed by the FIA and that obviously hasn't been respected either."